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| Home > Publications database > Investigation of GeSn as Novel Group IV Semiconductor for Electronic Applications |
| Book/Dissertation / PhD Thesis | FZJ-2018-02130 |
2018
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-95806-312-9
Please use a persistent id in citations: http://hdl.handle.net/2128/17929 urn:nbn:de:0001-2018050980
Abstract: Within the last few years single crystalline GeSn semiconductor alloys aroused significant scientific interest, especially since 2015, when GeSn with sufficiently high Sn content and crystalline quality was demonstrated as fundamentally direct bandgap group IV semiconductor. While enhanced optical properties are evident for direct bandgap materials compared to the fundamentally indirect Ge and Si group IV semiconductors, also enhanced electrical properties like increased carrier mobilities and enhanced band-to-band tunneling are expected for direct bandgap GeSn which are beneficial for metal-oxide semiconductor transistors and tunnel field-effect transistors, respectively. The novel GeSn semiconductor alloys thereby manifests a fascinating emerging material system allowing a wide scope to study its fundamental physical, electrical, optical and chemical properties. On the other hand the novelty of the material system demands the re-development or modification and verification of all steps necessary to build GeSn based semiconductor devices. A comprehensive study is presented, focusing on the electrical properties of GeSn, their dependence on Sn content and possible applications in novel electronic devices. The building blocks of field-effect transistors are studied individually. GeSn surface composition and manipulation are investigated $\textit{via}$ X-ray photoemission spectroscopy to study pre-high-$\kappa$ deposition cleaning and highly selective Ge/GeSn etching processes. NiGeSn alloys for the use as electrical contacts of GeSn devices are structurally and electrically characterized using X-ray diffraction, transmission electron microscopy and temperature dependent current voltage measurements, respectively. Schottky barrier height, sheet resistance and specific contact resistivity are extracted. The modification of the NiGeSn/GeSn Schottky barrier height $\textit{via}$ dopant segregation is demonstrated for the first time. Schottky-barrier heights as low as 0.06 eV are observed. As a next module metal-oxide-semiconductor capacitors are comprehensively studied. High-$\kappa$/GeSn interface trap densities are extracted for a wide range Sn contents. The focus is placed on the effect of the electronic band structure of GeSn on the capacitance voltage characteristics. Fundamental trends demonstrating the correlation of Sn-induced bandgap shrinkage and minority carrier response are observed. Furthermore a maximum capacitance of approx. 3 $\mu$F/cm$^{2}$ is achieved. As a step towards GeSn based tunnel field-effect transistors, Esaki diodes (tunnel diodes) are fabricated and electrically characterized. Negative differential resistance with a peak-to-valley current ratio of 2.3 is observed as an experimental proof of band-to-band tunneling. Enhanced band-to-band tunneling rates are observed in Ge$_{0.89}$Sn$_{0.11}$ $\textit{p-i-n}$ diodes compared to Ge taking advantage of the low and direct bandgap. These studies lead to the realization of vertical heterojunction Ge$_{0.93}$Sn$_{0.07}$/Ge tunnel field-effect transistors. An extensive analysis is provided identifying the various contributions to the overall transistor current, particularly band-to-band tunneling and trap-assisted tunneling. Finally, Hall measurements are presented, showing enhanced electron mobilities in direct bandgap GeSn as compared to Ge. With up to 4600 cm$^{2}$/Vs this marks the highest bulk electron mobilities at the respective doping level of 2.9 · 10$^{17}$ cm$^{−3}$ in a group IV semiconductor so far.
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